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  1. #1
    Join Date
    Mar 2003
    Location
    Virgin Islands
    Posts
    19

    Cool

    Having problems getting my suction pressures up. Here's the scenario.
    I just recently took over a condo complex and while replacing condensing units I found the last service company talked the owners into three ton systems from two tons and didn't change the line sets. So I have a three ton running 140 feet on 5/8 line. 3/8 Liquid line until it goes into the building where it changes to 1/4 inch. Since these are rental units, the owners are cheap and don't want to spend what it would take to change out the systems back to 2 tons. As I see it, the evaps are starving. I can't get my pressures to balance out. Suction is running around mid 50's and the closer I get to sixty my head is going up over 325. One of my fellow tech's suggested changing out the fan to get more air across the coil but the air pressure is too great for ducts now. There is no way to pull new pipe, it's set in concrete walls for about 25 feet. Roughly 115 feet is exposed and I thought about changing it out to 7/8th but the 1/4 inch is still a problem. I thought about a receiver but
    it would use the refrigerant up faster than I could get it into it.

    Any Ideas?

    Here's the latest one I'm changing out next week. 5 ton air handler, 3 1/2 ton condensing unit and 5/8" & 3/8" line set. Putting in new 3 ton system and changing line set.

    The quality of work here would put your wall of shame to shame.

    Anyone want to come down to work in Paradise where the ocean is a cool 85 degrees and the ambient never gets below 70 degrees. Summer rarely gets over 100 but you get used to it. We have 3 A/C companies down here and I swear not one of the techs ever went to A/C school. Real third world.
    You can still get R-12 for $125.00 bucks a thirty pounder. EPA who? I could
    probably get rich dropping dimes to the EPA if they did their job. When you see someone vent 12 systems and the EPA does nothing. Oh well,
    Just another day in Paradise!


    Anyone have any experience with Mitsubishi's City Multi. I attended two schools last year and selling my first one next month. At 25 cents a KWH it really pays for itself fast. I sell a lot of Mitsubishi mini splits and it is good equipment. A lot of junk coming in from China.

    Any suggestions are greatly appreciated.

    Saillar

  2. #2

    Post

    Just a student asking qustions.
    How hot is it outside?
    Whats the high side pressure?
    Is the suction line insulated?

  3. #3
    Join Date
    Jan 2005
    Posts
    175
    C4

  4. #4
    Join Date
    Nov 2004
    Posts
    2,927

    What is your superheat? Subcooling?
    What type metering devise?

    Also a temperture reading on liquid line leaving condenser and another one near evaporator.That should confirm any pressure drop.

    Manufacturer will specify line sizes.


    Where is Paradise? I hope to go there someday...
    Sometimes there are compounding complexities of multiple variables that are not intuitively obvious

  5. #5
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,296
    Were only the condensing units bumped up to three tons and the evap coils left at two tons?

    System capacity is determined by the compressor/condenser, so if you have 3 tons outdoors on 2 tons indoors, you have a compressor that can draw down that evap faster at 3 tons than at two. At the same time, if you're using a piston metering device, the orifice size hasn't changed if you still have a two ton evap coil. You have a compressor with more mass flow rate capacity shoving liquid refrigerant toward a metering device that is sized for a reduced mass flow rate capacity. Hence you'll see a higher head pressure and a greater pressure differential between the liquid entering the MD and the saturated vapor pressure inside the evap.
    This pressure differential is caused by the suction action of the compressor lowering the pressure within the evaporator. If the compressor has a greater ability to lower this pressure, it will tend to make the evap run at a lower pressure overall. If you attempt to bring up the suction pressure by adding more refrigerant, you'll be stymied by high head pressure and a suction pressure that does not come up much (with a TXV it likely won't rise at all). Remember you still have a two ton piston orifice in the system!

    What kind of Delta T's are you getting across the two ton evaps with three ton condensers on them? Superheat and subcooling readings?

    Remember, pressures tell only part of the story.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  6. #6
    Join Date
    Apr 2003
    Location
    the Great Pacific Northwest
    Posts
    607
    Friction loss is friction loss, up sizing where possible would help.

    Quote from shophound,
    "You have a compressor with more mass flow rate capacity shoving liquid refrigerant toward a metering device that is sized for a reduced mass flow rate capacity. Hence you'll see a higher head pressure and a greater pressure differential between the liquid entering the MD and the saturated vapor pressure inside the evap"

    High head would most likely be from over charge or dirty condensers or other problem with condensers subcooling will tell the story otherwise you would see low head from low load of undersized evaps.

    how much sub cooling?

    what type of metering device?

    how much does paridise pay?

  7. #7
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,296
    Originally posted by hvac-tech-lane
    Friction loss is friction loss, up sizing where possible would help.


    High head would most likely be from over charge or dirty condensers or other problem with condensers subcooling will tell the story otherwise you would see low head from low load of undersized evaps.


    Agreed. I was going off the disclosed info that saillar kept goosing the system with gas, trying to get the suction pressure to rise. I suppose I was thinking "out loud" about what might happen in a mismatched system, exclusive of dirty coils, etc. From what else he describes, these systems have been hacked to death, and likely have dirty coils, non-condensibles, etc.

    I would tend to think that an undersized evap coil would tend to run at a lower pressure with an upsized condenser, all other airflows and cleanliness factors being as they should be. The load on the evap would be the same if the evap was sized properly for the building (same CFM/ton, same heat load within structure, etc), but the condenser would present extra pumping capacity, it being upsized. In this case it may be negated by overly long lineset size (friction loss) and dirty equipment.

    However, with a clean system that has an undersized coil, wouldn't the evap tend to run flooded, resulting in a low superheat, vs. starved?

    To saillar, get some superheat and subcooling readings off this equipment and post it, if you please.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  8. #8
    Join Date
    Apr 2003
    Location
    the Great Pacific Northwest
    Posts
    607

    [/B][/QUOTE]
    Quote from shophound

    However, with a clean system that has an undersized coil, wouldn't the evap tend to run flooded, resulting in a low superheat, vs. starved?

    To saillar, get some superheat and subcooling readings off this equipment and post it, if you please.
    [/B][/QUOTE]

    I would agree, provided the orfice or piston were matched to the condensing unit and the line sizes were right.

    I just wanted to clarify the high head issue as it relates to condensing capacity or loss thereof from the aforementioned factors, and that while installing the proper linesets is the best way to go the use of the existing section and upsizing as much of the accesible run may be the best solution availible.

    To saillar:
    Dont hold back on what needs to be done, propose the right solution at the right price for you. You're there now because the previous company must not have taken good care of them. People want a good value, and no mater how cheap the price if the system doesnt work they will feel riped off, however if you fix it right and the're happy they'll feel they got a good value. Perhaps you need to propose running exposed and let them hire someone else to build a cover. Above all have fun and keep us posted.

    [Edited by hvac-tech-lane on 02-10-2005 at 02:50 AM]

  9. #9
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,914
    I have run into things like that in condos too.

    You really can't charge the systems accuratly at all because you are getting huge pressure drops through the liquid line and the suction lines.

    The actual pressure in the evaperator coil is likely much higher than you are reading outside at the condensor.
    Likewise, the liquid line pressure is much lower just before the meetering device than it is at the outdoor unit.

    By adding refrigerant to the system to try and bring your suction pressure up at the outdoor unit, you are grossly overcharging the system.

    You can probably get them cooling decently if you put a pressure tap in the suction line leaving the indoor unit and charge to the correct superheat based on the pressure and temperature readings you get near the outlet to the coil.

    It isn't ideal, but if they don't want to correct the problems, it is the only way to charge the units.

    The main issues you will have with this is that the pressure drop in the lines greatly reduces the systems capacity, and the compressor will be a little starved for cooling. I think starving the compressor for cooling a little is less destructive to it than juicing the charge up though.
    If more government is the answer, then it's a really stupid question.

  10. #10
    Join Date
    Mar 2003
    Location
    Virgin Islands
    Posts
    19
    The systems started out as Rheem 2 ton splits with fixed metering vs. TXV's.
    All systems in question got new AH's & CU's upgraded to Rheem 3 tons, fixed orifice. As for condition of equipment, new condensing unit, new or just cleaned evap's. Good air flow. I don't have the Superheat or Subcooling #'s
    handy but will reply with them. Rheem spec's 3/4" & 3/8 line size to 50 feet and suction jumps up to 7/8. All 2 ton systems that were not upgraded and with deterioating condenser coils are running close to normal.

    One area that I am investigating is possible clogs. Many of the systems don't have any filter driers, and may have scale being picked up in the oil.

    As for where is Paradise? St Thomas, Virgin Islands. What does it pay? The other companies advertise $8.00 - 12.00/hr for a seasoned tech where I pay $25.00/hr. Service call rate is between $50.00 - $100.00 per man. I'm on the lower end of the scale. Customers pay a lot for hack work. Here's an example:My duct man needed a duculator, went down to the biggest competitor to buy one. Owner says, what is a ductulator? There attitude is if it's a 6 ton space then 12 will work better. In bidding situations where I came in at half the dollars and half the equip size, they thought I didn't know what I was doing.
    After I did the job and cooling well at half the size, they were sure pleased.

    I appreciate your tips, I can only talk to myself for so long.

  11. #11
    Join Date
    Apr 2004
    Posts
    7,680
    You better look at your liquid line. Rheem doesn't specify 3/8 liquid line in 2 or 3 ton systems. Also you need to revisit the length of line... Are these HPs or a/c? Is the outdoor unit above or below the indoor unit and by how much?

    Lol... for 2 weeks a year (my vacation) I'll come down and work for ya, heck for 25 bucks an hour, airfare and hotel accomidations why not. (j/k but I wish I wasn't)

  12. #12
    Join Date
    Jul 2000
    Location
    NW IL.
    Posts
    3,935
    5/8th inch on a 3 ton suction line seems undersized. Check the installation instructions. If you measure the temperature of the suction line as it leaves the evaporator and at the condensing unit I am sure you are going to have a temperature drop at the condensing unit. Did they change the evaporator coil?

  13. #13
    Join Date
    Sep 2002
    Posts
    3,708
    How will you ever get the charge right any way with a three
    ton system on a two ton distribution system?

    I guess in paradise there used to jet noises.


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